1. Field of the Invention
The present invention relates to an adaptive feedback control system and method for controlling electrical heating of an element and maintaining constant resistance operation thereof, specifically to a gas-sensing system and method for determining presence and concentration of a target gas species based on the amount of adjustment required for maintaining an electrical gas sensor element at a constant electrical resistance.
2. Description of the Related Art
Combustion-based gas sensors comprising heated noble metal filaments are widely used for detecting the presence and concentration of a combustible gas species of interest. Catalytic combustion of such gas species is induced on the surface of such heated noble metal filaments, resulting in detectable changes in the temperature of such filaments. Each gas sensor usually comprises a matching pair of filaments: a first filament—known as the detector—actively catalyzes combustion of the target gas species and causes temperature changes, and a second filament—known as the compensator—does not contain the catalytic material and therefore only passively compensates for changes in the ambient conditions. When such pair of filaments is incorporated into a Wheatstone-Bridge circuit, an out-of-balance signal can be produced to indicate the presence of the target gas species.
Because it is often desirable to operate the combustion-based gas sensors at a prescribed temperature so as to maintain a known, constant rate of combustion, the conventional gas sensors utilize a feedback control circuit for adjusting the electrical power supplied to the heated noble metal filaments to compensate for the temperate changes caused by combustion. In other words, the more heat generated by the combustion, the more adjustment is required to maintain the constant temperature operation, and the less heat generated by the combustion, the less adjustment is required. In such manner, the presence as well as concentration of the gas species can be determined based on the amount of adjustment required for maintaining the detector and the compensator at constant temperatures (i.e., if no adjustment is required, then there is no target gas species present; the greater the adjustment required, the higher the concentration of such gas species).
Because the temperature of a metal filament directly impacts its electrical resistance, which can be precisely measured by various electrical devices, the feedback control circuit used by the conventional gas sensors usually provides an electrical resistance setpoint (Rs) as an input (r), and monitors the electrical resistances (R) of the metal filament as an output (c) indicative of temperature changes in such filament, while the output electrical resistance (R) is also used as a feedback signal for adjusting the electrical current passed through the filament to compensate for any temperature changes detected. Specifically, the differences between such input set point resistance (Rs) and the feedback signal of the output electrical resistance (R) are recorded as an error signal (e=Rs−R), on the basis of which a control signal (u) is determined and used for manipulating the electrical power supplied to the metal filaments so as to reduce the error signal (e).
The well-known proportion-integral-derivative (PID) feedback control system determines the control signal (u) as a function of the error signal (e), which contains three terms including (1) a proportional term (KP×e), (2) an integral term (KI×∫e(t)dt), and (3) a derivative term
      (                  K        D            ×                        ⅆ          e                          ⅆ          t                      )    .The proportional term (KP×e) is proportional to the error signal (e), where KP is its proportionality constant. The integral term (KI×∫e(t)dt) is proportional to the time integral of the error signal (e), where KI is its proportionality constant. The derivative term
  (            K      D        ×                  ⅆ        e                    ⅆ        t              )is proportional to the time derivative of the error signal (e), where KD is its proportionality constant.
A major drawback and limitation of the conventional PID feedback control system lies in the need to empirically tune the proportionality constants (KP, Kl, and KD) for each controlled element at a specific set of operating conditions, since optimal values of such proportionality constants vary significantly from element to element and at various operating conditions. Therefore, whenever the controlled elements or the operating conditions change, such proportionally constants (KP, KI, and KD) have to be re-tuned. When such PID feedback control system is used for controlling the combustion-based gas sensors, in which addition/removal/replacement of sensor elements are frequent and the operating conditions constantly change due to fluctuations in gas concentration, pressure, temperature, humidity, etc., the task of re-tuning becomes labor-intensive and cumbersome.
It is therefore an object of the present invention to provide a feedback control system and method for maintaining constant resistance operation of combustion-based gas sensors, which is adaptive to variations in the sensor elements and in the operating conditions and requires minimum or no re-tuning when the sensor elements or the operating conditions change.
It is also an object of the present invention to provide an adaptive feedback control system and method for maintaining constant resistance operation of electrically heated elements in general.
Other aspects, features and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.